In wireless communication systems, such as digital radio or television transmission, an information signal is communicated from a transmitter to a receiver as multiple signals via a channel comprising several independent paths. These multiple signals are called multipath signals and the channel is called a multipath channel. Because of the complex addition of multipath signals, the overall signal strength at a receiver will vary. The phenomenon of received signal strength variation due to complex addition of multipath signals is known as "fading".
A channel encoder (also known as a "channel coder") or similar device can be employed to compensate for fast fading. If the signal strength at a receiver "fades" slowly, however, a receiver experiencing a low signal strength, called a "deep fade", will observe a weak signal strength for a longer period of time than can be readily compensated for using a channel coder. Slow fading is a particular problem in car radio receivers.
Two types of channel coding systems are block coding and convolutional coding. Slow fading may cause a burst of incorrect data symbols at a data receiver. If the burst of incorrect data symbols is short enough the channel coder can detect or correct it. However, when fading is too slow, long bursts of errors can occur which cannot be adequately corrected and unacceptable performance results.
Interleaving/deinterleaving with a channel coder can be used to further combat slow fading. An interleaver at the transmitter rearranges a set of data symbols in a pseudorandom fashion and a deinterleaver at the receiver rearranges the symbols in the original order. However, interleaving/deinterleaving with a channel coder is not sufficient to combat fading if the deep fades last for long enough periods of time.
Spatial diversity can also be used to combat slow fading. Spatial diversity involves the use of a plurality of receiving and/or transmitting antennas. If two receiving antennas, for example, are spatially diverse from one another, the two signals received at the individual antennas will have independent fading characteristics and can be combined to reduce the probability of deep fades, independent of the fading rate.
Unfortunately, spatial diversity may require wide spacing of receiver antennas, typically at least a quarter wavelength. Thus, the receive antennas must be separated by at least 2.5 feet for audio broadcasting at 100 MHz, which is impossible to achieve in small portable receiving systems.
Spatial diversity may also be achieved using multiple transmit antennas. However, transmitting the same signal out of each transmit antenna is not useful, as it just generates more multipath signals at the receiver. One technique previously proposed is to use channel coding with interleaving/deinterleaving in combination with a time varying phase offset between each antenna as proposed in U.S. patent application, Ser. No. 07/890,977, filed on May 29, 1992 to Weerackody which is incorporated herein by reference. This time varying offset creates rapid fading at the receiver antenna, which can be compensated by channel coding with interleaving. For this technique to be effective, however, the signals received from the multiple transmit antennas must be independent. Unfortunately, in digital audio broadcasting (DAB) for example, the transmit antennas are usually very high, e.g., on top of the World Trade Center, to provide wide area coverage. At such heights spacing of tens of wavelengths between the transmit antennas is required to insure substantially independent fading. At 100 MHz with digital audio broadcasting, the required spacing is therefore in excess of hundreds of feet, which is not generally practical.